Characterization of the count rate performance of modern gamma cameras

Abstract

Purpose: Evaluation of count rate performance (CRP) is an integral component of gamma camera quality assurance and system deadtime (τ ) may be utilized for image correction in quantitative studies. This work characterizes the CRP of three modern gamma cameras and estimates τ using two established methods (decay and dual source) under a variety of experimental conditions. Methods: For the decay method, uncollimated detectors were exposed to a Tc-99m source of relatively high activity and count rates were sampled regularly over 48 h. Input count rate at each time point was based on the lowest observed count rate data point. The input count rate was plotted against the observed count rate and fit via least-squares to the paralyzable detector model (PDM) to estimate τ (rates method). A novel expression for observed counts as a function of measurement time interval was derived, taking into account the PDM and the presence of background but making no assumption regarding input count rate. The observed counts were fit via least-squares to this novel expression to estimate τ (counts method). Correlation and Bland-Altman analyses were performed to assess agreement in estimates of τ between the rates and counts methods. The dependence of τ on energy window definition and incident energy spectrum were characterized. The dual source method was also used to estimate τ and its agreement with estimates from the decay method under identical conditions was also investigated. The dependences of τ on the total activity and the ratio of source activities were characterized. Results: The observed CRP curves for each gamma camera agreed with the PDM at low count rates but deviated substantially from it at high count rates. The estimates of τ determined from the paralyzable portion of the CPR curve using the rates method and the counts method were found to be highly correlated (r = 0.999) but with a small (∼6%) difference. No statistically significant difference was observed between the estimates of τ using the decay or dual source methods under identical experimental conditions (p = 0.13). Estimates of τ increased as a power-law function with decreasing ratio of counts in the photopeak to the total counts. Also, estimates of τ increased linearly as spectral effective energy decreased. No significant difference was observed between the dependences of τ on energy window definition or incident spectrum between the decay and dual source methods. Estimates of τ using the dual source method varied as a quadratic on the ratio of the single source to combined source activities and linearly with total activity. Conclusions: The CRP curves for three modern gamma camera models have been characterized, demonstrating unexpected behavior that necessitates the determination of both τ and maximum count rate to fully characterize the CRP curve. τ was estimated under a variety of experimental conditions, based on which guidelines for the performance of CRP testing in a clinical setting have been proposed.

Comprehensive SPECT/CT system characterization and calibration for 177Lu quantitative SPECT (QSPECT) with dead-time correction

“…Our data clearly shows that the paralysable model based on wide-spectrum acquisition count rate is nevertheless able to accurately estimate lost primary photon counts regardless of the cause, up to the maximum usable wide-spectrum count rate (~350 kcps) of the system. As observed by others, the actual maximum count rate was lower than the theoretical one (1/e • 0.550 μs = 669 kcps for our system), which emphasizes the need to experimentally measure it to fully characterize the system [16]. Beyond that point quantification is not possible and, with our SPECT/CT system, the image is distorted because of the divergent detector behaviour (Fig.…”

“…Several methods have been proposed to evaluate these parameters, some more demanding than others in terms of acquisition time, decay period, and image processing [8,9,[16][17][18][19][20][21]. Planar imaging-based calibration is faster and more convenient to execute and is expected to yield accurate CF [9,20] relative to fully tomographic calibration as the reference method [9,21,22].…”

Practical reconstruction protocol for quantitative 90Y bremsstrahlung SPECT/CT

“…If, however, correction for deadtime count loss is needed, the recently proposed revised monitor source method for practical deadtime count loss compensation can be employed. 29,30…”

“…31 Acrylic slabs of various thicknesses (2, 5, 9, 12, and 16 cm) were placed right below the phantom to introduce attenuation and scatter. For the 5 cmthick acrylic slab, the distance between the collimator and the phantom was varied to 15,20,25,30, and 35 cm. Planar static images were acquired for each imaging condition.…”

Factors affecting the stability and repeatability of gamma camera calibration for quantitative imaging applications based on a retrospective review of clinical data

“…According to the results, camera sensitivity variations were small and had little effect on the variance of the calibration factor. The extrinsic flood source count rate was corrected for dead time using a paralyzable model, while current SPECT cameras are neither purely paralyzable nor non-paralyzable [16]. It is possible that errors due to this correction masked the usefulness of the sensitivity factor to explain variations in the calibration factor.…”